The Color change in Vivianite and its effect on the Optical Properties

Vivianite, the hydrous phosphate of ferrous iron, is generally described as colorless when pure and unaltered, but in most of its known occurrences it is green to blue, the latter color being probably the most common. The streak is at first colorless to faint bluish white, but rapidly changes to deep blue on exposure. Reduction of the mineral to powder by grinding likewise produces a rapid change in color, from colorless to deep blue. This change in color of vivianite has been ascribed to oxidation of a part of the iron to the ferric condition, since analyses of the strongly colored varieties of the mineral invariably show the presence of ferric oxide in quantity ranging up to 33 per cent, or more.